Passive infrared detectors known in the art have an optical design which is usually based on the use of multiple optical elements, such as lens or mirror segments, arranged in one or more rows, each row including one or more segments. The segments within the rows are arranged with their optical axes spread azimuthally in a plane, generally parallel to the horizontal, or inclined with respect to the horizontal. Each of the segments is arranged to focus IR energy emanating from a pre-defined detection zone onto an infrared sensor such as a pyroelectric sensor, which is common to multiple segments. The combined detection zones of the multiple optical elements or segments, constitute the field-of-view of the detector, which is defined as the detection region covered by the detector or the “coverage” of the detector.
A commercially successful prior art detector is the Coral Plus detector, commercially available from Visonic Ltd. of Israel. This detector includes a lens assembly and a dual element pyroelectric sensor, comprising a pair of sensor elements. The pyroelectric sensor employed in this detector is a Perkin-Elmer LHi-968 dual element sensor. The lens assembly includes multiple Fresnel lens segments arranged in three rows, which are positioned in front of the sensor and serve as a detector window. Other prior art lens assemblies comprise only two rows of optical segments, an upper row, including Fresnel lens segments, and a lower row, including cylindrical optical segments.
A person moving through the field-of-view of the detector causes generation of a signal output from the sensor. This signal is defined to be a “desired signal”. Signal processing circuitry of the detector detects and processes the desired signal and activates an alarm signal output.
There are also known detectors which operate similarly to the lens-based detectors described hereinabove but employ mirror segments rather than lens segments. In such detectors, incoming infrared radiation enters the detector through a wide IR transparent window in the detector housing and is reflected by the mirror segments to focus onto a pyroelectric sensor. The window is provided to prevent insects and other spurious matter from entering the detector. In lens-based detectors of the type described hereinabove, the lens itself also functions as a window.
The prior art detectors described hereinabove, whether lens-based, mirror-based or employing both lenses and mirrors, are particularly suitable for indoor applications. However, when installed outdoors or in harsh environments, such detectors are subject to operational conditions of various types, which cause false alarms. These conditions may include, but are not limited to:                Wind bursts which produce flows of hot or cold air onto and into the detector and cause a change in the temperatures of various elements of the detector, such as the housing, the window or the sensor.        Rain and snow which cause changes in the temperature of the background as well as changes in the temperature of the housing and the window. These effects are amplified when the detector is also subject to wind.        Extreme environmental conditions and extreme changes thereof which cause significant thermal interference signals.        Large variations in temperature within the field-of-view. For instance, a black asphalt road that is exposed to the sun can reach temperatures as high as 50° C. -60° C. while a nearby pool or irrigated grass can have temperatures as low as 15° C. In such cases, a moving person having a temperature of 35° C.-37° C. will differ from the background by over +20° C. with respect to the irrigated grass and −15° C. with respect to the asphalt road.        Movement of background elements in the field-of-view.        Fast changes in background temperature within the field-of-view.        High level of sunlight radiation.        Presence of animals, such as pets and rodents.        
In prior art PIR detectors, the pyroelectric sensor receives not only “desired signals” but also simultaneously receives undesired thermal interference signals (“undesired signals”) emanating concurrently from within the field-of-view.
Thus, for example, if a detector, having nine lens or mirror segments in a horizontal row which define nine detection zones, is designed to detect “desired signals” emanating from a single detection zone at a given moment in time, it actually receives at the same time also “undesired signals” emanating concurrently from the eight remaining detection zones. The “undesired signals” result from temperature variations at the wide detector window and the housing, air drafts, moving trees and bushes, animals and other sources as described above.
Accordingly, in this example, the total level of the “undesired signals” (interference) to which the detector is exposed, is about nine times larger than the “desired signal” emanating from a single zone. In many cases, especially in outdoor environments, the total level of the “undesired signals” may be even larger than that of the “desired signal” which the detector is designed to detect. This is the main reason for the many false alarms in outdoor environments.
Various solutions have been proposed for outdoor applications by manufacturers such as Optex Co. Ltd. of Japan, Crow Electronic Engineering Ltd. of Israel, and Paradox Security Systems Ltd. of Canada. Generally, the proposed solutions incorporate two sensors, having generally overlapping fields-of-view, into the same detector housing to activate a common alarm output upon generally simultaneous detection of motion by both sensors. Prevention of false alarms is based on the statistical assumption that the probability that each of the two sensors will generate a false alarm at approximately the same instant is very low. On the other hand, inasmuch as both detectors have more or less the same field-of-view, detection processing is based on detecting a “desired signal” in both sensors at approximately the same time. Although such detectors perform better outdoors than a detector employing a single sensor, they still do not provide sufficiently reliable detection, because much of the interference existing outdoors, as explained hereinabove, generates “undesired signals” simultaneously in both sensors, due inter-alia, to the fact that both sensors view the same field-of-view.
The following published patent documents and other publications are believed to represent the current state of the art:
U.S. Pat. Nos. 3,524,180; 3,958,118; 4,058,726; 4,081,680; 4,087,688; 4,271,359; 4,375,034; 4,479,056; 4,604,524; 4,614,938; 4,645,930; 4,704,533; 4,709,152; 4,912,748; 4,943,800; 5,296,707; 5,559,496; 5,693,943; 5,703,368; 5,844,240; 6,150,658; 6,163,025 and 6,211,522.
Product sheets of known outdoor detectors on the market:                Optex Co. Ltd.—models LX-402/802N and VX-402/402R/402REC. Model VX-402/402R/402REC is described in U.S. Pat. No. 5,703,368.        Crow Electronic Engineering Ltd.—D&D (Daredevil) and MRX-300. The MRX-300 incorporates a micro-wave detector.        Paradox Security Systems Ltd.—Digigard DG85.        